1. Field of the Invention
The present invention relates to a digital to analog (D-A) converter for converting a digital signal to an analog signal and utilizing the analog signal to drive a load.
2. Description of the Prior Art
FIG. 7 is a circuit diagram showing a driving circuit for a load where a prior art D-A converter is used. As shown in FIG. 7, an N-bit digital signal DATA is taken in a D-A converting block 1. The D-A converting block 1 converts the digital signal DATA to analog voltage V.sub.IN.
The analog voltage V.sub.IN is applied to a negative input of an operational amplifier 2 via a node 4 and a resistance 101. The negative input of the operational amplifier 2 is connected to a load 3 via resistances 104 and 105 while a positive input of the operational amplifier 2 is grounded via a resistance 102 and also connected to the load 3 via a resistance 103. The resistances 101 through 104 all have a common resistance value r, and the resistance 105 has a resistance value R.sub.0.
In such a structure, applying the N-bit digital signal DATA to the D-A converting block 1, the D-A converting block 1 converts the digital signal DATA to the analog voltage V.sub.IN to output it to the node 4.
At this time, assuming that an amplification factor of the operational amplifier 2 is A.sub.V, output voltage V.sub.01 of the operational amplifier 2 can be obtained according to the following Formula 1: ##EQU1##
Formula 1 is transformed to obtain Formula 2: ##EQU2##
Then, a formula expressing a relation between voltages V.sub.0 and V.sub.01 at opposite ends of the resistance 105 will be set up. Assuming now that an impedance of the load 3 is Z, Formula 3 is obtained as follows: ##EQU3##
Then, substituting Formula 3 into Formula 2, V.sub.0 is expressed as in Formula 4: ##EQU4##
Rearranging Formula 4, Formula 5 is obtained: ##EQU5##
On the other hand, load current I.sub.0 flowing in the load 3 is expressed as in Formula 6: ##EQU6##
Substituting Formula 5 into Formula 6, the load current I.sub.0 is expressed as in Formula 7: ##EQU7##
Assuming now that r&gt;&gt;R.sub.0 and A.sub.V &gt;&gt;Z are satisfied, Formula 7 is simplified into Formula 8: ##EQU8##
Thus, regardless of the impedance Z of the load 3, the load current I.sub.0 can be determined.
In this way, the D-A converting block 1 converts the digital signal DATA to the analog voltage V.sub.IN, which, in turn, the operational amplifier 2 converts into the load current I.sub.0 regardless of the impedance Z of the load 3 to supply it to the load 3. That is, the digital signal DATA is utilized to drive the load 3.
However, when a frequency of the D-A converted analog voltage is close to a GB (gain-bandwidth) product of the operational amplifier, the amplification factor A.sub.V of the operational amplifier takes a value close to 1. Hence, A.sub.V &gt;&gt;Z is not satisfied, which results in Formula 8 not being exact, and therefore, the load current I.sub.0 is affected by the impedance Z of the load 3.
The prior art D-A converter is structured as mentioned above, and as shown in FIG. 8, it is provided with a single D-A converting block 1 for converting the N-bit digital data DATA. This is why N bits are needed for the D-A converting bit of the D-A converting block 1.
FIG. 9 is a graph revealing problems of the prior art D-A converter shown in FIGS. 7 and 8. In FIG. 9, although an ideal load current I.sub.0 is expressed by Curve L1, a V-I converting characteristic of a voltage to current converter (V-I converter) 20 causes an actual curve related to a reference current I.sub.00 to deviate to a plus side (Curve L2) or deviate to a minus side (Curve L3). In such a case, a leading phase shift results (t.sub.01 -t.sub.00) when the actual curve deviates to the plus side, or otherwise a lagging phase shift results (t.sub.00 -t.sub.02) when it deviates to the minus side. That is, in the prior art D-A converter, because of the V-I converting characteristic of the V-I converter, there arises a phase difference in its load current.